8.3 Site investigation & foundation considerations

Site Investigation and Foundation Considerations

Before design, the engineering geologist characterizes the subsurface - its materials, groundwater, and hazards - so foundations can be sized safely and economically. This subsurface exploration is the bridge between geology and geotechnical engineering.

The site investigation program

A typical investigation proceeds from general to specific:

1. Desk study and reconnaissance - review geologic maps, aerial photos, prior borings, and seismic and flood data; walk the site to note landforms, seeps, and distress in existing structures.
2. Subsurface exploration - advance borings and test pits to sample soil and rock and to log stratigraphy. Test pits expose shallow conditions directly.
3. Sampling - disturbed samples (from a split-spoon sampler) preserve composition for classification; undisturbed samples (thin-walled Shelby tubes pushed into cohesive soil) preserve structure for strength and consolidation testing. Rock is recovered by core drilling, and RQD is logged.
4. In-situ and laboratory testing quantify strength, compressibility, and index properties.

Standard Penetration Test and in-situ methods

The Standard Penetration Test (SPT) is the most common in-situ test. A 140-lb hammer falling 30 inches drives a split-spoon sampler; the N-value is the number of blows required to drive the sampler the final 12 inches (300 mm) of an 18-inch drive. N correlates with the relative density of sands and, more roughly, the consistency of clays, and it feeds liquefaction and bearing-capacity analyses (corrected to N1,60). The cone penetration test (CPT) pushes an instrumented cone for a continuous strength profile. Geophysical methods - seismic refraction (depth to rock, rippability), electrical resistivity, ground-penetrating radar (GPR), and MASW (shear-wave velocity for seismic site class) - provide non-intrusive coverage between borings. Piezometers and observation wells define the groundwater table, which controls effective stress and liquefaction potential.

Foundation types

Foundations transfer structural loads to the ground. The choice depends on load magnitude and on the depth to competent bearing material:

• Shallow (spread) foundations - isolated spread footings, continuous strip footings, and mat (raft) foundations - are used when strong soil lies near the surface. They spread load over a bearing area just below the structure.
• Deep foundations - piles (driven) and drilled shafts / caissons (bored) - are used when near-surface soils are weak, loose, expansive, collapsible, or liquefiable, and the load must be carried down to firmer strata. Deep elements resist load through end bearing (the tip resting on strong soil or rock) and skin (side) friction along the shaft.

Bearing capacity and settlement

Two independent criteria govern shallow-foundation performance:

• Bearing capacity is the soil's ability to support load without shear failure. The ultimate bearing capacity (Terzaghi and later theories) is reduced by a factor of safety (commonly about 3) to an allowable bearing capacity used in design. It increases with soil strength (c and phi), footing width, and depth.
• Settlement is downward movement under load and must stay within tolerable limits. Immediate (elastic) settlement occurs at once; primary consolidation settlement - the slow squeezing of water from saturated fine-grained clays - is time-dependent and analyzed with Terzaghi consolidation theory; secondary compression (creep) follows. Differential settlement (uneven movement between footings) is more damaging than uniform settlement because it distorts and cracks the structural frame.

Geologic constraints on construction

Beyond capacity and settlement, the geologist flags conditions that constrain or forbid construction:

• Active faults and strong-ground-motion (seismic) hazard, including surface-rupture setbacks.
• Expansive, collapsible, and liquefiable soils, which may require ground improvement, moisture control, or deep foundations.
• Karst and sinkhole terrain with hidden voids.
• Unstable slopes and old landslide deposits.
• Shallow or aggressive groundwater (corrosive or sulfate-bearing soils attack concrete and steel) and swelling shales.
• Rippability and excavatability of rock, estimated from seismic velocity.

Ground improvement and the geologic report

Where poor ground cannot be avoided, engineers improve it. Compaction and dynamic (drop-weight) compaction densify loose fills and sands; stone columns and vibro-compaction mitigate liquefaction; surcharge preloading with wick drains accelerates the consolidation of soft clay; grouting fills karst voids and cements loose soil; and soil mixing or lime/cement stabilization treats expansive and weak soils. Boring depth and spacing are chosen to reach below the zone of significant stress influence and to bracket variability across the site. The investigation culminates in a geotechnical / engineering-geologic report presenting boring logs, a stratigraphic cross-section, laboratory results, groundwater data, a hazard assessment, and specific foundation and earthwork recommendations. That report - and the professional judgment behind it - is the deliverable the PG exam ultimately tests.

Investigation checklist

Matching the foundation to the ground - and identifying the hazards that rule some options out - is the practical payoff of engineering geology, where the fundamentals and hazards from the previous sections come together.